The introduction of Wi-Fi™ calling has given subscribers of a carrier network the ability to make calls in areas where they don't have cell coverage by means of an available Wi-Fi access point (AP). The Wi-Fi AP can be used by the subscribers' user equipment (UE) to access the carrier network using Wi-Fi calling in lieu of voice over Long Term Evolution (VoLTE), which uses a cell site to access the carrier network. Despite this benefit, an attempt to utilize Wi-Fi calling may sometimes result in the carrier network prohibiting service to the UE, such as when the subscriber's location cannot be ascertained. One reason, among others, for prohibiting service when the subscriber's location cannot be ascertained is for purposes of calling 911. For example, federal regulations specify that a subscriber is to have a reasonable expectation that he/she can dial 911 and be routed to the correct (i.e., nearest) public safety answering point, which depends on the carrier being able to ascertain the subscriber's location. Thus, in a situation where a subscriber's 911 location is not ascertainable, the subscriber's UE, in attempting to subscribe for registration status (e.g., by sending a Session Initiation Protocol (SIP) request using the SUBSCRIBE method), may receive a subscription error (e.g., 403 forbidden), or a SIP response using the NOTIFY method to specify a terminated registration status. In either case, the result is that service is prohibited for the subscriber (e.g., prohibiting voice calling service over Wi-Fi).
The manner in which existing UEs handle such errors is suboptimal. For instance, the existing 3rd Generation Partnership Project (3GPP) standards specify that a UE, in encountering issues during subscription for registration status, should immediately attempt to register for service again in an effort to provide service to the subscriber. However, for the reasons noted above, a missing 911 location constitutes an “unrecoverable” error in the sense that the issue will never be resolved as long as the 911 location remains unascertainable, regardless of how many times the UE attempts to register for service. Thus, a UE, following 3GPP standards, may be caught in a continuous loop of registration failures when 911 location is missing. This is suboptimal because, when a high number of UEs are engaged in similar registration attempts, a large amount of network traffic is created, causing large-scale network bandwidth reduction. In some instances, a large number of UEs may be continuously sending, and re-sending, registration attempts to the carrier network, which can result in a complete network outage.
In addition to a missing 911 location, other issues, such as high network congestion, may arise when a UE is attempting to subscribe for registration status, which can prevent a UE from successfully registering for service. Some of these other types of issues, unlike the missing 911 location, may be “recoverable” in the sense that, given enough time, the issue will eventually resolve itself (e.g., network congestion will eventually subside), and a UE may eventually succeed in registering for service upon resolution of the issue. The existing 3GPP 24.229 specification describes that the network, in some instances, can transmit a response (e.g., a 503 response) to a failed registration attempt with a “Retry-After” header field value indicating a wait time (typically 300 seconds) that the UE should wait before attempting a next registration. However, these 3GPP UE procedures are not optimized to resolve some recoverable issues, such as high network congestion. For example, when network congestion is particularly high, many UEs can be engaged in re-registration attempts at the same time (due to immediate retry attempts or retries attempted at the same, fixed intervals), which does not provide enough time for the congestion in the network to subside. Thus, existing UE procedures for handling errors during subscription for registration status may cause recoverable errors to persist, thereby preventing service for potentially large numbers of UEs.